The present invention relates to control of a synchronous motor which uses a permanent magnet in its field system. In particular, the present invention relates to a control apparatus and a control method which implement the control of the synchronous motor without using a sensor to detect the magnetic pole position of the rotor.
As techniques for controlling the synchronous motor without detecting the magnetic pole position of the rotor, there are techniques disclosed in JP-A-2006-158101 and JP-A-2007-174721.
According to the technique disclosed in JP-A-2006-158101, a minute changes is given to each of voltage commands on a dc axis which is an estimation magnetic pole axis of a synchronous motor and a qc axis perpendicular to the dc axis, and the magnetic pole position of the synchronous motor is directly estimated by utilizing a difference between a positive side current value and a negative side current value of a resultant current pulsating component appearing on a DC bus or a difference in current change rate of the current pulsating component between the positive side and the negative side.
The technique disclosed in JP-A-2006-158101 utilizes magnetic saturation characteristics of the iron core to estimate the magnetic pole position. It is now supposed that a high frequency alternating voltage is applied onto a d-axis which is the direction of the rotor magnetic flux (magnetic flux of a permanent magnet) in the synchronous motor. If a current flows in a direction which strengthens the magnet magnetic pole, then the magnetic flux saturates and the inductance decreases, and consequently the current change quantity (ΔI+) becomes great. Conversely, if a current flows in a direction which weakens the magnet magnetic flux, then the magnetic flux decreases and the inductance increases or becomes constant, and consequently the current change quantity (ΔI−) becomes less than ΔI+ described above. If an alternating current which changes according to whether the polarity is positive or negative is injected onto the d-axis, therefore, a current in which a waveform on the positive side is asymmetric to that on the negative side flows.
According to the technique disclosed in JP-A-2006-158101, an alternating voltage is applied to two orthogonal phase angles and the above-described asymmetric current characteristics are observed from DC bus current detected values. Since the magnetic pole position of the rotor is not known, the alternating voltage is applied at an arbitrary phase angle. Asymmetric current characteristics caused at that time change depending upon the magnetic pole position of the rotor. In JP-A-2006-158101, it is supposed that the asymmetric current characteristics change according to a sin-function or a cos-function of the magnetic pole position of the rotor and the magnetic pole position of the rotor is calculated by using an arctangent function. According to the technique disclosed in JP-A-2006-158101, the magnetic pole position of the rotor can be estimated with a high precision without being affected by a difference in the structure of the synchronous motor such as the salient pole or the non-salient pole.
The technique disclosed in JP-A-2007-174721 also utilizes magnetic saturation characteristics of the iron core to estimate the magnetic pole position of the iron core. According to the technique disclosed in JP-A-2007-174721, a positive-negative alternating voltage is applied to each of three-phase rotor winding axes and a phase current on an axis with the alternating voltage applied thereto is detected. If positive phase current detected values corresponding to the three phases are obtained, then they are compared in magnitude to search for a phase current which assumes a maximum value and the magnetic pole position of the rotor is deduced with a resolution of 120 degrees. In addition, in order to make a decision as to a detailed magnetic pole position of the rotor, a current difference magnification α, which will be described later, is calculated and compared with characteristic data (data representing a relation between the rotor position and the magnification α) measured beforehand.
Recognizing the positive phase current detected values described above as maximum current, intermediate current and minimum current, it is disclosed that the magnification α is calculated by using Equation (1). According to the technique disclosed in JP-A-2007-174721, it is not necessary to prepare a large number of complicated computation expressions beforehand and the rotor position can be estimated easily with high precision.Magnification α=|maximum current−intermediate current|/|intermediate current−minimum current|  (1)